Thermal microphotonic sensor and sensor array

ABSTRACT

A thermal microphotonic sensor is disclosed for detecting infrared radiation using heat generated by the infrared radiation to shift the resonant frequency of an optical resonator (e.g. a ring resonator) to which the heat is coupled. The shift in the resonant frequency can be determined from light in an optical waveguide which is evanescently coupled to the optical resonator. An infrared absorber can be provided on the optical waveguide either as a coating or as a plate to aid in absorption of the infrared radiation. In some cases, a vertical resonant cavity can be formed about the infrared absorber to further increase the absorption of the infrared radiation. The sensor can be formed as a single device, or as an array for imaging the infrared radiation.

GOVERNMENT RIGHTS

This invention was made with Government support under Contract No.DE-AC04-94AL85000 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates in general to sensing of infraredradiation, and in particular to a thermal microphotonic sensor whichsenses infrared radiation by using heat produced by the infraredradiation to change a coupling of light between an optical waveguide andan optical resonator. The thermal microphotonic sensor can be formed asan individual sensor, or as a sensor array which is useful for infraredimaging applications.

BACKGROUND OF THE INVENTION

Infrared imaging is useful for detecting electromagnetic radiation atwavelengths beyond that which is visible to the human eye. Infraredimaging has applications for detecting people and/or machines by theiremitted heat which is a form of infrared radiation. Such infraredimaging can be performed at night or when clouds or smoke wouldotherwise obscure normal vision. Infrared imaging is also important toprovide detailed thermal images from space, or from a high altitudeusing an airplane or an unmanned aerial vehicle (UAV).

Many different types of infrared sensors are known in the art includingbolometers and quantum detectors. Quantum detectors such as mercurycadmium telluride (MCT) detectors are highly sensitive but requirecooling down to cryogenic temperatures. The cooling of MCT detectorsconsumes considerable electrical power and typically requires acryostat. For satellite and UAV applications, the power consumption andweight of cryogenically-cooled quantum detectors can be limiting factorswhich prevent a scaling of quantum detectors to larger sizes.

Over the past decade, resistive bolometers have made significant inroadsin room-temperature infrared imaging. However, in order to achieve asufficient level of sensitivity, the resistive bolometers need tooperate over an entire range of 7-14 μm where the blackbody emissionpeak lies at room temperature. Additionally, resistive bolometers do notenable sufficient standoff for many remote infrared sensingapplications. Resistive bolometer array sizes currently remain below onemegapixel which significantly limits resolution and the field of viewfor these devices.

What is needed is an infrared sensor and infrared sensor array thatoperates without cryogenic cooling and which provides a sensitivityhigher than that of current resistive bolometers and preferablyapproaching or even exceeding that of conventional quantum detectors.

The present invention addresses this need by providing a thermalmicrophotonic sensor and sensor array which utilizes heat provided byincident infrared radiation to change a coupling of light between one ormore optical waveguides and optical resonators. Operation of the thermalmicrophotonic sensor and sensor array of the present inventiontheoretically can be more sensitive than other available types ofuncooled detectors including resistive bolometers

These and other advantages of the present invention will become evidentto those skilled in the art.

SUMMARY OF THE INVENTION

The present invention relates to a thermal microphotonic sensor fordetecting infrared radiation. The thermal microphotonic sensor comprisesan optical resonator suspended above a substrate on a support post byone or more tethers which are connected between the optical resonatorand the support post, with the optical resonator having a resonantfrequency which changes (i.e. shifts in frequency) in response toheating of the optical resonator by the infrared radiation. An opticalwaveguide is located proximate to the optical resonator to couple lightfrom the optical waveguide into the optical resonator, and to transmit aremainder of the light not coupled into the optical resonator throughthe optical waveguide to a photodetector. The photodetector generates anelectrical output signal which is proportional to an intensity (i.e.absorbed power) of the infrared radiation. The light can be provided bya laser.

In some embodiments of the present invention, the light from the lasercan be passed through an optical modulator to amplitude modulate thelight. This modulation generates a pair of frequency-tunable sidebandson either side of an optical carrier frequency of the light. The opticalcarrier frequency is the frequency of the light from the laser which isinput into the optical modulator and modulated to generate thefrequency-tunable sidebands. The frequency of the sidebands is thustunable over a predetermined frequency range, which can encompass theresonant frequency of the optical resonator and any shift in theresonant frequency due to heating by the incident infrared radiation,using a sinusoidal electrical input signal to the optical modulator.

A narrowband optical filter can be optionally provided to blocktransmission of the optical carrier frequency and one frequency-tunablesideband of the light, and to transmit the other frequency-tunablesideband of the light. The narrowband optical filter can be used tofilter the light prior to entry into the optical waveguide, or to filterthe light after it exits from the optical waveguide.

The optical resonator can comprise a ring resonator (also sometimesreferred to as a micro-ring resonator). In some embodiments of thepresent invention, the optical resonator can comprise monocrystallinesilicon; and the support post and each tether can comprise siliconoxide. In other embodiments of the present invention, the opticalresonator and each tether can comprise silicon nitride; and the supportpost can comprise silicon oxide.

The optical resonator can also include an infrared absorber on theoptical resonator. The infrared absorber can comprise aninfrared-absorbing coating covering at least a part of a surface of theoptical resonator. Alternately, the infrared absorber can comprise aninfrared-absorbing plate located on or above the optical resonator. Theinfrared absorber is thermally coupled to the optical resonator to heatthe optical resonator in response to incident infrared radiation.

In certain embodiments of the present invention, a vertical resonantcavity can also be formed about the infrared absorber to enhanceabsorption of the infrared radiation. The vertical resonant cavity canbe formed with a first mirror located on a top surface of theinfrared-absorbing plate, and a second mirror located on the substratebeneath the optical resonator. The first mirror can in some casescomprise a metal screen.

In some embodiments of the present invention, a plurality of the tetherscan be interconnected by one or more concentric rings which are locatedbetween the optical resonator and the support post. Interconnecting thetethers with these rings is useful to increase the thermal isolationbetween the optical resonator and the substrate.

The present invention also relates to a thermal microphotonic sensorwhich comprises one or more optical resonators supported on a substratewith an infrared absorber supported on each optical resonator andthermally coupled thereto, and with an optical waveguide being locatedproximate to each optical resonator to provide a coupling of lightbetween the optical waveguide and that optical resonator. The couplingof the light for each optical resonator is dependent upon heating ofthat optical resonator in response to infrared radiation which isincident onto the infrared absorber located on that optical resonator.Means are also provided in the thermal microphotonic sensor for sensingthe coupling of the light between the optical waveguide and each opticalresonator to provide an indication of an intensity of the infraredradiation which is incident onto each infrared-absorbing plate.

The infrared absorber can be an infrared-absorbing coating located on asurface of each optical resonator. Alternately, the infrared absorbercan be an infrared-absorbing plate supported on each optical resonator.In either case, heating of the infrared absorber associated with eachoptical resonator is produced by absorption of the infrared radiation,with the heating then being thermally coupled into that opticalresonator to shift the resonant frequency therein. This shift in theresonant frequency of each optical resonator is then determined via itseffect on the coupling of the light between the optical waveguide andeach optical resonator.

The means for sensing the coupling of the light between the opticalwaveguide and each optical resonator can comprise a photodetector tosense the light emitted from an output end of each optical waveguide.The means for sensing the coupling of the light between the opticalwaveguide and each optical resonator can also comprise a laser toprovide the light with a fixed frequency, or with a tunable frequency.With a fixed-frequency laser, the shift in the resonant frequency ofeach optical resonator can be determined by a change in magnitude of thedetected light at the output end of each waveguide. With a tunablefrequency laser, the shift in the resonant frequency of each opticalresonator can be determined by measuring a characteristic curve of theresonator, or alternately by scanning (i.e. tuning) the laser at a fixedrate and measuring the times when laser is scanned to particularreference points on either side of the characteristic curve of theresonator.

In some embodiments of the present invention, a vertical resonant cavitycan be formed about the infrared-absorbing plate, with the verticalresonant cavity being resonant at a wavelength of the infraredradiation. This can be useful to increase the absorption of the infraredradiation by the infrared-absorbing plate at a particular wavelength.

Each optical resonator generally comprises a ring resonator. Eachoptical resonator, which is coupled to the same optical waveguide, alsopreferably has a different resonant frequency in the absence of anyheating thereof. The light, which is coupled between each opticalwaveguide and a plurality of optical resonators can comprise a pluralityof different frequencies to provide for coupling of one frequency of thelight to each optical resonator. Each different frequency of the lightcan also be made tunable over a range of frequencies. The light, whichcan be provided by one more lasers, is coupled into an input end of theoptical waveguide.

The present invention further relates to a thermal microphotonic sensorwhich comprises a substrate having at least one optical waveguide formedthereon to transmit light from an input end of each optical waveguide toan output end thereof; at least one optical resonator suspended abovethe substrate proximate to each optical waveguide to couple the lighttherebetween when a frequency of the light is near a resonant frequencyfor that optical resonator; an infrared absorber on each opticalresonator to absorb incident infrared radiation and produce heat, withthe heat from the infrared absorber being coupled into the opticalresonator to change the resonant frequency therein; and a photodetectorlocated proximate to the output end of the optical waveguide to detectthe light and to generate an electrical output signal containinginformation about an intensity of the infrared radiation incident ontothe infrared absorber on each optical resonator.

The resonant frequency of each optical resonator, which is coupled tothe same optical waveguide, will generally be different. Also, the lightcan comprise a plurality of different frequencies to provide forcoupling of one frequency of the light to each optical resonator. Insome embodiments of the present invention, each different frequency ofthe light can be provided by a separate laser, with the plurality ofdifferent frequencies of the light then being combined together in anoptical multiplexer (e.g. a dense wavelength division multiplexer). Anoptical modulator can be used to amplitude modulate each differentfrequency of the provided by each separate laser to generate a pair offrequency-tunable sidebands on either side of each different frequencyof the light.

In some embodiments of the present invention when the thermalmicrophotonic sensor is used to form an infrared focal plane array(FPA), an optical element (e.g. a lens or mirror) can be provided toimage the infrared radiation onto each infrared absorber. In theseembodiments of the present invention, an information processor can alsobe provided to receive the electrical output signal from eachphotodetector and to generate therefrom an image of the infraredradiation which can be stored and/or displayed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate several aspects of the presentinvention and, together with the description, serve to explain theprinciples of the invention. The drawings are only for the purpose ofillustrating preferred embodiments of the invention and are not to beconstrued as limiting the invention. In the drawings:

FIG. 1 shows a schematic perspective view of a first example of athermal microphotonic sensor according to the present invention.

FIG. 2 shows characteristic curves of an optical resonator withoutheating by infrared radiation (curve “A”), and the same resonator whichis shifted in frequency due to heating by infrared radiation (curve“B”).

FIGS. 3A-3F show schematic cross-section views along the section line1-1 in FIG. 1 to illustrate fabrication of the device of FIG. 1.

FIG. 4 shows a response curve for the thermal microphotonic sensor ofFIG. 1 with infrared radiation at a wavelength of 10 μm being repeatedlyswitched “on” and “off.”

FIG. 5 shows a schematic perspective view of a second example of thethermal microphotonic sensor of the present invention.

FIG. 6 shows a schematic plan view of a third example of the thermalmicrophotonic sensor of the present invention.

FIGS. 7A-7F show schematic cross-section views along the section line2-2 in FIG. 6 to illustrate fabrication of the device of FIG. 6.

FIGS. 7G-7I show schematic cross-section views along the section line2-2 in FIG. 6 to illustrate additional process steps which can be usedto add an infrared-absorbing plate to the device of FIG. 6.

FIG. 8 shows a schematic perspective view of a fourth example of thethermal microphotonic sensor of the present invention.

FIGS. 9A-9I show schematic cross-section views along the section line3-3 in FIG. 8 to illustrate fabrication of the device of FIG. 8.

FIG. 10 shows a schematic diagram of a first example of a sensor arrayformed according to the present invention.

FIG. 11 shows a schematic diagram of a second example of a sensor arrayformed according to the present invention.

FIG. 12 shows a schematic representation of a frequency spectrum at anoutput of the optical modulator to illustrate the generation of a pairof tunable frequency sidebands which are generated on either side of acenter frequency of the light from each laser due to modulation of thelight from each laser.

FIG. 13 shows a schematic diagram of a third example of a sensor arrayformed according to the present invention.

FIG. 14 shows the amplified signal in the sensor array of FIG. 13 as thesideband frequency of the light is scanned across the characteristiccurve of a heated resonator to illustrate how the frequency shift of theresonator can be determined from the times t₀ and t₁ when thecharacteristic curve intersects a reference point (indicated by thehorizontal dashed line).

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown a schematic perspective view of afirst example of a thermal microphotonic sensor 10 of the presentinvention. The apparatus 10 comprises an optical resonator 12 (alsoreferred to as an optical cavity, a microcavity, or a microringresonator). The optical resonator 12 is suspended on a support post 14above a substrate 16 by a plurality of tethers 18 which are connectedbetween the optical resonator 12 and the support post 14. An opticalwaveguide 20 is located near a periphery of the optical resonator 12 toallow an evanescent coupling of light 22 between the optical waveguide20 and the optical resonator 12.

The light 22 can be provided by a laser 24 which can be, for example, asingle-frequency semiconductor laser such as a distributed Braggreflector (DBR) laser operating at a wavelength of about 1.5 microns(μm). The light 22 from the laser 24 can be coupled into the opticalwaveguide 20 at an input end 26 thereof and is transmitted through theoptical waveguide 20 to an output end 28 of the waveguide 20 where thelight 22 is detected by a photodetector 30. The light 22 can be coupledinto and out of the optical waveguide 20 using lenses and/or opticalfibers which are not shown in FIG. 1.

In the example of FIG. 1, the light 22 in the optical waveguide 20 willbe evanescently coupled into the optical resonator 12 when a frequencyf_(L) of the light 22, which is inversely proportional to the wavelengthof the light 22, is near a resonant frequency f₀ of the opticalresonator 12. The optical resonator 12 in the example of FIG. 1 is aring resonator (also termed a microring resonator) in which a portion22′ of the light 22 which is evanescently coupled into the resonator 12circulates around the periphery of the resonator 12 as illustrated bythe curved arrow in FIG. 1 by being either waveguided or by reflectingoff an outside edge of the resonator 12 in a whispering-gallery mode.The term “ring resonator” as used herein is intended to include opticalresonators having a circular shape as shown in the example of FIG. 1 orhaving a polygonal shape (e.g. a square or rectangular shape), anelliptical shape, or oval shape (also termed a racetrack shape). Thoseskilled in the art will further understand that, although the portion22′ is shown as circulating in a clockwise direction, the portion 22′can circulate in a counterclockwise direction if the optical waveguide20 were to be located on an opposite side of the resonator, or if thelight 22 in the waveguide 20 were to propagate in an opposite directionto that shown in FIG. 1.

The coupling of the light 22 between the optical waveguide 20 and theoptical resonator 12 will depend upon the exact frequency f_(L) of thelight 22 in the optical waveguide 20 relative to the resonant frequencyf₀ of the optical resonator 12. This can be understood with reference toFIG. 2. In FIG. 2 as the frequency f_(L) of the light 22 in the opticalwaveguide 20 is varied about the resonant frequency f₀, a characteristiccurve is generated for the light 22 transmitted through the opticalwaveguide 20 and detected with the photodetector 30. This characteristiccurve, which is labelled “A” in FIG. 2, has an inverse Lorentzian shapewith a full-width at half maximum (FWHM) which depends upon a qualityfactor Q of the optical resonator 12. When the frequency f_(L) of thelight 22 in the optical waveguide 20 is tuned to coincide with theresonant frequency f₀, a minimum amount of the light 22 will be detectedby the photodetector 30 since substantially all of the light 22 is beingcoupled into the optical resonator 12. Detuning the frequency f_(L) ofthe light 22 away from the resonant frequency f₀ reduces the couplingbetween the optical waveguide 20 and resonator 12, thereby increasingthe amount of light detected by the photodetector 30 as shown in FIG. 2.

If the frequency f_(L) of the light 22 is fixed at a particularreference point on the characteristic curve labelled “A” in FIG. 2, thenany change in the resonant frequency f₀ of the optical resonator 12 dueto temperature can be measured simply by monitoring the amount of thelight 22 exiting the optical waveguide 20 using the photodetector 30.This can be seen, for example, with the frequency f_(L) being set at a50% transmission point on the characteristic curve “A” as indicated bythe downward-pointing arrow in FIG. 2. Any change in the resonantfrequency f₀ of the optical resonator 12 will shift the characteristiccurve “A” and this will result in a change in the amount of light 22transmitted through the waveguide 20 and detected by the photodetector30. The change in the resonant frequency f₀ can occur from directheating of the optical resonator 12 due to incident infrared radiation100. An interior portion of the optical resonator 12, which is notnecessary for propagation of the light 22′ in the resonator 12, can beleft in place as shown in FIG. 1 to increase the volume of materialwhich absorbs the incident infrared radiation 100, thereby increasingthe heating of the resonator 12.

In other embodiments of the present invention, an infrared absorber canbe provided in the device 10 to absorb the infrared radiation 100producing heating therein which can then be thermally coupled from theinfrared absorber to the optical resonator 12 to shift the resonantfrequency therein. The infrared absorber can comprise aninfrared-absorbing coating 46 on a surface of the optical resonator asshown in FIG. 5, or an infrared-absorbing plate 60 supported on theoptical resonator 12 as shown in FIGS. 7I and 8. This shift in thecharacteristic curve for the resonator 12 with heating due to theinfrared radiation 100 can be on the order of 1-10 GHz/° C., and isillustrated in FIG. 2 by the curve labelled “B.” Here, a change inintensity ΔI of the light 22 which is detected by the photodetector 30is indicated by a double-headed arrow. The term “infrared radiation” asused herein is defined as being electromagnetic radiation with awavelength that is in a range of 0.7 μm to 1000 μm.

Although not shown in FIG. 1, a reference optical resonator can beoptionally provided in the thermal microphotonic sensor. The referenceoptical resonator, which can be formed substantially identically to theoptical resonator except for being shielded from heating by the infraredradiation is useful to frequency stabilize the laser 24 by locking it toa particular point on the characteristic curve of the reference opticalresonator. This can be done by incorporating the reference opticalresonator into a feedback loop which stabilizes the frequency of thelaser 24. The reference optical resonator can also provide a referencefor measuring the shift in the resonant frequency of the opticalresonator 12 due to heating by the sensed infrared radiation.

The thermal microphotonic sensor 10 in FIG. 1 can be formed usingconventional semiconductor micromachining processes which are well knownin the art. This is schematically illustrated by a series ofcross-section views in FIGS. 3A-3F which are taken along the sectionline 1-1 in FIG. 1 during various steps in the fabrication of thethermal microphotonic sensor 10. Those skilled in the art willunderstand that only the essential processing steps are illustrated inFIGS. 3A-3F. Many other processing steps, which are well-known to thoseskilled in semiconductor micromachining, have been omitted includingsteps for mask formation and removal, photolithography, and cleaning.

In FIG. 3A, a substrate 16 is provided on which are blanket deposited alayer 32 of silicon oxide and a layer of silicon nitride 34. Thesubstrate 16 can comprise, for example, a semiconductor such as silicon(e.g. a silicon wafer, or a portion thereof). The layers 32 and 34 canbe deposited by chemical vapor deposition (CVD) at a temperature in therange of 450-750° C. depending upon whether the layers 32 and 34 aredeposited by plasma-enhanced CVD or low-pressure CVD. The silicon oxidelayer 32 can also be formed as a thermal oxide in which a portion of asilicon substrate 16 is thermally oxidized and converted into silicondioxide by exposure of the silicon substrate 16 to an oxygen or moisture(e.g. steam) at a high temperature of about 1050° C. In some cases, thesilicon oxide layer can comprise a silicate glass such as TEOS which canbe deposited by CVD from the thermal decomposition of tetraethylorthosilicate. Thus, the term “silicon oxide” as used herein is intendedto include silicon dioxide (SiO₂) and silicate glasses such as TEOS.

After the deposition of each layer 32 and 34, a chemical-mechanicalpolishing (CMP) step can be used to precisely adjust the thickness ofthe layer 32 or 34 and also to provide a smooth surface for the layer 32or 34. This provides smooth top and bottom surfaces for the siliconnitride layer 34 which can reduce a scattering loss of the light 22 inthe optical waveguide 20 and in the optical resonator 12 which will beformed from the silicon nitride layer 34. The thickness of the siliconoxide layer 32 can be, for example, 2-5 μm; and the thickness of thesilicon nitride layer 34 can be, for example, 0.2-0.3 μm.

In FIG. 3B, the silicon nitride layer 34 can be patterned by providing aphotolithographically-defined etch mask 36 over the silicon nitridelayer 34 with a shape and openings 38 to define the optical waveguide20, the optical resonator 12, the tethers 18 and a top 40 for thesupport post 14 which will be formed later. To photolithographicallydefine the various elements formed in the silicon nitride layer 34, adeep UV photoresist can be used with a bottom anti-reflection coating(BARC). The deep UV photoresist allows submicron features to be definedin the silicon nitride layer 34, including a submicron air gap formedbetween the waveguide 20 and the resonator 12.

The sidewalls of the photoresist openings 38 can be smoothed, if needed,to produce smoother sidewalls for the optical waveguide 20 and theoptical resonator 12 to reduce a scattering loss for the light 22. Anadditional smoothing step can be used to smooth the sidewalls of thewaveguide 20 and the resonator 12 after these elements are formed byetching any asperites in the silicon nitride layer 34. Photoresist andsilicon nitride smoothing techniques, which can be used to fabricate thethermal microphotonic sensor 10 of the present invention, are well knownin the art (see e.g. M. J. Shaw, et al., “Fabrication Techniques for LowLoss Silicon Nitride Waveguides,” Proceedings of the Society ofPhotoOptical Instrumentation Engineers (SPIE), vol. 5720, pp. 109-118,2005). Smoothing of the photoresist can be performed, for example, byetching the sidewalls of the openings 38 using an hydrogenbromide/oxygen (HBr—O₂) plasma. Smoothing of the silicon nitridewaveguide 20 and resonator 12 can be performed by wet etching in a hotphosphoric acid bath at 180° C., or alternately by exposing thewaveguide 20 and resonator 12 to steam at a temperature of 1050° C. tooxidize any asperites in the silicon nitride followed by etching withhydrofluoric acid (HF) to remove the oxidized asperites.

In FIG. 3C, to define the various elements 12, 18, 20 and 40 beingformed from the silicon nitride layer 34, a reactive ion etching (RIE)step can be used. The RIE step etches away portions of the siliconnitride layer 34 which are exposed by the etch mask 36.

The optical waveguide 20 can be, for example, about 1 μm wide, with theoptical resonator 12 being, for example, 5-100 μm in diameter. Thetethers 18 can be, for example, 0.05-0.2 μm wide; and the top 40 for thesupport post 14 can be up to a few microns wide. A spacing between theoptical waveguide 20 and the optical resonator 12 can be, for example,about 0.2 μm.

After etching the silicon nitride layer 34 to form the various elements12, 18, 20 and 40, an annealing step can be performed at a hightemperature of 1050-1200° C. for up to several hours. A separateannealing step can also be performed to anneal the silicon oxide layer32 prior to deposition of the silicon nitride layer 34. These annealingsteps can improve the light transmission in the optical waveguide 20 andalso in the optical resonator 12 by reducing the presence of molecularH—O bonds in the silicon oxide layer 32 beneath the waveguide 20 and byreducing molecular H—N and Si—H bonds in the silicon nitride used toform the waveguide 20 and the resonator 12. These molecular bonds, ifnot reduced, will absorb some of the light 22 to reduce the transmissionin the waveguide 20 and also to degrade a quality factor, Q, of theresonator 12.

In FIG. 3D, a second etch mask 42 can be formed from the deep UVphotoresist with an annular opening 44 which is centered about theperiphery of the optical resonator 12.

In FIG. 3E, the silicon oxide layer 32 can be etched away beneath theoptical resonator 12, the tethers 18 and the optical waveguide 20 usingan isotropic wet etchant comprising HF which is introduced through theannular opening 44 by immersing the substrate 16 into the HF etchant.The HF etching step can be timed to leave a portion of the silicon oxidelayer 32 in place beneath the support post 40 to complete the supportpost 14. Some of the silicon oxide layer 32 is also left in place tosupport the optical waveguide 20 which is now suspended above thesubstrate 16 in the vicinity of the optical resonator 12 as shown inFIG. 1. The silicon substrate 16 and the various elements 12, 18, 20 and40 formed from the silicon nitride layer 34 are not substantiallychemically attacked by the HF etchant.

In FIG. 3F, the second etch mask 42 is removed. This leaves the opticalresonator 12 suspended above the substrate 16 by the support post 14 andtethers 18 as shown in FIG. 1.

FIG. 4 shows a response curve for the thermal microphotonic sensor 10 ofFIG. 1 using about 1 microWatt (pW) of 10.6-μm infrared radiation 100from a carbon dioxide laser 24 incident onto a top surface of a25-μm-diameter optical resonator 12. The 10.6-μm infrared radiation 100is absorbed into the silicon nitride material forming the resonator 12.The silicon nitride is highly absorptive to infrared radiation atwavelengths in the 8-12 μm range and is, at the same time, transparentto guide the light 22 from the laser 24 at a wavelength of 1.508 μm inboth the optical waveguide 20 and the resonator 12.

In FIG. 4, the incident 10.6-μm infrared radiation 100 is repeatedlyswitched “on” and “off” at a 1 millisecond repetition rate using a lightchopper. This shifts the resonant frequency f₀ of the optical resonator12 back and forth as the resonator 12 repeatedly heats up and cools downin response to the incident 10.6-μm infrared radiation 100. The light 22at the photodetector 30 is at a maximum when the infrared radiation 100is switched “off” and is at a minimum when the infrared radiation 100 isswitched “on.” This response curve in FIG. 4 shows the utility of thedevice 10 for detecting the infrared radiation 100 in the 8-12 μm range.

The absorption of the 10.6-μm infrared radiation 100 by the 0.2-μm-thicksilicon nitride resonator 12 used to obtain the response curve of FIG. 4is estimated to be on the order of 10%, with the resonator Q being about10⁴, and with the thermal isolation of the resonator 12 from thesubstrate 16 being limited by conduction through the air so that thethermal conductance G is about 10⁻⁶ W-K⁻¹. The sensitivity of the device10 of FIG. 1 can be further improved by operating the device 10 undervacuum, by increasing the Q of the resonator 12, and by increasing theabsorption of the infrared radiation 100.

The absorption of the infrared radiation 100 can be increased using aninfrared absorber which is tailored to highly absorb infrared radiation100 over a particular wavelength range. The infrared absorber cancomprise an infrared-absorbing coating 46 covering at least a part ofthe optical resonator 12. This is schematically illustrated in theperspective view of FIG. 5 which shows a second example of a thermalmicrophotonic sensor 10 formed according to the present invention.

The infrared-absorbing coating 46 in the device 10 of FIG. 5 cancomprise a material such as a metal (e.g. tungsten) which can bedeposited over the optical resonator 12 during fabrication of the device10. Virtually any material can be used for the infrared-absorbingcoating 46 which has a relatively high absorption of the infraredradiation 100 over a particular wavelength range of interest and whichcan be deposited in a relatively thin layer on the order of about 1 μmor less. The infrared-absorbing coating 46 can be deposited by anymethod known to the art including evaporation, sputtering, CVD, spinningonto the substrate 16, ink-jet deposition, etc. The infrared-absorbingcoating 46 can be patterned during deposition (e.g. using a shadowmask), or can be patterned after deposition (e.g. by lift-off or by anRIE step). The infrared-absorbing coating 46 can have a size and shapeto enhance the absorption of the infrared radiation 100 at a particularwavelength, or wavelength range. The infrared-absorbing coating 46 ispreferably omitted from an outer edge of the resonator 12 as shown inFIG. 5 to prevent any attenuation or scattering of the portion 22′ ofthe light 22 circulating in the resonator 12 due to the presence of thecoating 46.

Although the tethers 18 in the first and second examples of the presentinvention extend radially outward from the support post 14, in otherembodiments of the present invention, a more circuitous thermal path canbe used to further increase the thermal isolation of the opticalresonator 12 from the substrate 12. FIG. 6 shows a schematic plan viewof a third example of the thermal microphotonic sensor 10 of the presentinvention in which a plurality of tethers 18 are interconnected with oneor more concentric rings 48 to provide the more circuitous thermal pathbetween the optical resonator 12 on the support post 14.

In the example of FIG. 6, the optical waveguide 20 and the opticalresonator 12 both comprise monocrystalline silicon; and the tethers 18and the concentric rings 48 comprise silicon dioxide. The use of silicondioxide for the tethers 18 and concentric rings 48 together with themore circuitous thermal path provided by these elements 18 and 48 cangreatly increase the thermal isolation of the optical resonator 12 fromthe substrate 16. Calculations with an ANSYS finite element thermalmodel indicate that a thermal conductance G=1.2×10⁻⁸ W-K⁻¹ can beattained for a 10-μm-diameter resonator 12 using a series ofinterconnected silicon dioxide tethers 18 and silicon dioxide concentricrings 48 which are each 0.05 μm wide and 0.25 μm thick.

Fabrication of the device 10 of FIG. 6 will now be described withreference to FIGS. 7A-7I which show a schematic cross-section viewsalong the section line 2-2 in FIG. 6.

In FIG. 7A, a substrate 16 can be provided which comprises acommercially-available silicon-on-insulator substrate having amonocrystalline silicon body 50 and a monocrystalline silicon layer 52sandwiched about a silicon dioxide layer 54. The monocrystalline siliconlayer 52 can be, for example, 0.2-0.3 μm thick; and the silicon dioxidelayer 54 can be, for example, 1-5 μm thick.

In FIG. 7B, a photolithographically-defined etch mask 36 can be formedon the monocrystalline silicon layer 52. The etch mask 36 can be aspreviously described with reference to FIG. 3B. A reactive ion etching(RIE) step can then be used to pattern the monocrystalline silicon layer52 to define the waveguide 20, resonator 12, tethers 18, concentricrings 48 and the top 40 for the support post 14. The etch mask 36 canthen be removed to leave the various elements 12, 18, 20, 40 and 48formed from the monocrystalline silicon layer 40 as shown in FIG. 7C.

In FIG. 7D, a second etch mask 42 can be provided over the substrate 16as previously described with reference to FIG. 3D. The second etch mask42 includes an annular opening 44 which is centered about the peripheryof the optical resonator 12. A plurality of additional openings 44′ canalso be provided between the resonator 12 and the top 40 of the supportpost 14 to provide a faster etching time for removing the silicondioxide layer 54.

In FIG. 7E, the silicon dioxide layer 54 can be etched away beneath thevarious elements 12, 18, 20 and 48 as previously described withreference to FIG. 3E using the isotropic HF etchant. The etching can betimed and stopped when the silicon dioxide layer 54 is laterally etchedback to the top 40 of the support post 14 or beyond as shown in FIG. 7E.This completes the formation of the support post 14. A part of thesilicon dioxide layer 54 is also left in place to suspend the opticalwaveguide 20 as shown in FIG. 6.

In FIG. 7F, the second etch mask 42 is removed. The substrate 16containing the optical resonator 12 suspended above the substrate 16 bythe support post 14, tethers 18 and concentric rings 48 can then beannealed in an oxygen ambient at a high temperature of 900-1000° C. forsufficient time to completely oxidize the monocrystalline silicon in the50-nanometers-wide tethers 18 and concentric rings 48, therebyconverting these elements to silicon dioxide. This high-temperatureoxidation step also converts an exposed outer portion of the opticalwaveguide 20 and the optical resonator 12 to silicon dioxide, with thesilicon dioxide being about 25 nanometers thick. This forms a silicondioxide cladding 56 over a monocrystalline silicon core 58 for the boththe waveguide 20 and the resonator 12 which reduces a transmission lossof the light 22 in both of these elements.

In the example of FIG. 6, the optical resonator 12 can be directlyheated by the incident infrared radiation 100 which is absorbed by themonocrystalline silicon and silicon dioxide materials forming theresonator 12. Alternately, an infrared absorber can be provided in theapparatus 10 to absorb the incident infrared radiation 100 and totransfer the resultant heat generated in the infrared absorber to theoptical resonator 12. This can be done using an infrared-absorbing plate60 which can be supported on the optical resonator 12. The addition ofthe infrared-absorbing plate 60 can be provided, for example, after thestep of FIG. 7C. This can be done by initially blanket depositing alayer 62 of silicon dioxide or a silicate glass such as TEOS over thepatterned elements 12, 20, 40 and 48 formed from the monocrystallinesilicon layer 52 as shown in FIG. 7G. The layer 62 can be, for example,1-2 μm thick. After blanket deposition of the layer 62 by CVD, the layer62 can be planarized by a CMP step. A plurality of openings 64 can thenbe etched down through the layer 62 to expose a portion of the opticalresonator 12.

In FIG. 7H, a layer 66 of silicon nitride can be deposited by CVD tofill in the openings 64, thereby forming legs 68 which will support theinfrared-absorbing plate 60 on the resonator 12. The silicon nitridelayer 66, which can have a thickness of, for example, 0.2-1 μm, can thenbe patterned by an RIE etch step to form the infrared-absorbing plate60. The shape of the infrared-absorbing plate 60 can be circular,polygonal, elliptical, oval, or any arbitrary shape. In some cases, theshape of the infrared-absorbing plate 60 can be selected to provide anenhanced absorption of the infrared radiation 100 at a particularwavelength or wavelength range. As an example, the infrared-absorbingplate or a metal coating thereon can be shaped to form a bow-tie antennaor a patch antenna to receive the infrared radiation 100 at a particularwavelength determined by the size of the antenna. A ground plane (notshown) can be formed below the antenna by using a doped substrate 16, oralternately by providing a metal layer (e.g. tungsten) on the substrate16 beneath the antenna (see FIG. 8).

After depositing the silicon nitride layer 66, a plurality of openings70 can be etched through the silicon nitride layer 66 and around theinfrared-absorbing plate 60 during formation of the plate 60 as shown inFIG. 7H. This can be done with an RIE step. The openings 70 will allowthe infrared-absorbing plate 60 and a remainder of the silicon nitridelayer 66 to be used as an etch mask for removing the underlying layers54 and 62 using the HF etchant. The number and shape of the openings 70can be selected to allow the HF etchant to completely remove the layers54 and 62 surrounding the resonator 12, the tethers 18, the waveguide 20and the concentric rings 48 while leaving a portion of the layers 54 and62 in place to form the support post 14 and to support the opticalwaveguide 20 and the remainder of the silicon nitride layer 66. This isshown in FIG. 7I. The remainder of the silicon nitride layer 66 can beleft in place in the completed device 10.

A high-temperature oxidation step in an oxygen ambient can then beperformed as previously described with reference to FIG. 7F to oxidizethe monocrystalline silicon in the tethers 18 and concentric rings 48and thereby convert these elements into silicon dioxide. This greatlyreduces the thermal conductivity of these elements 18 and 48 since thethermal conductivity of silicon dioxide is two orders of magnitude lowerthan that for monocrystalline silicon. As previously discussed, thishigh-temperature oxidation step also forms a thin (e.g. 25 nm) silicondioxide cladding 56 over both the waveguide 20 and the resonator 12 byconverting a portion of the monocrystalline silicon material in theseelements 20 and 12 into silicon dioxide.

In other embodiments of the present invention, the optical resonator 12can be provided with a plurality of tabs 72 (see FIG. 8) extendingradially inward from the resonator 12, with the infrared-absorbing plate60 being supported on the tabs 72. These tabs 72 can be formed from themonocrystalline silicon layer 52; or alternately they can be formed fromdeposited silicon nitride (e.g. during formation of the legs 68).

FIG. 8 shows a schematic perspective view of a fourth example of thethermal microphotonic sensor 10 of the present invention. In the exampleof FIG. 8, the optical waveguide 20 comprises silicon nitride and issupported above the substrate 16 on a silicon oxide base 74. A siliconoxide support post 14 is also used to suspend the optical resonator 12above the substrate 16, which can comprise silicon. In this example ofthe present invention, a silicon nitride infrared-absorbing plate 60 issupported on the resonator 12 by a plurality of legs 68 which areattached to silicon nitride tabs 72 extending inward from the opticalresonator 12.

In the example of FIG. 8, a partially-transmitting mirror 76, which cancomprise a metal screen as shown in FIG. 8 or alternately a thin metallayer, is provided over the infrared-absorbing plate 60; and anothermirror 78 is located on the substrate 16 beneath the optical resonator12. The mirror 76 can comprise, for example, aluminum; and the mirror 78can comprise, for example, tungsten. Both of these metals are resistantto chemical attack by the HF etchant. The thickness of each mirror 76and 78 can be, for example, 0.1-0.2 μm. The size of the openings in themetal screen used to form the partially-transmitting mirror 76 can beselected to provide a predetermined value for the transmission of thismirror 76 at a particular wavelength of interest for the infraredradiation 100. As an example, the openings in the metal screen can beabout 3 μm square to detect infrared radiation 100 over a wavelengthrange of 8-12 μm for which the silicon nitride plate 60 and the siliconnitride optical resonator 12 are both strongly absorbing. Theinfrared-absorbing plate 60 can be, for example, about 20 μm square and0.2 μm thick.

The mirrors 76 and 78 form a vertical resonant cavity about theinfrared-absorbing plate 60 that is useful to reflect the infraredradiation 100 back and forth one or more times between the two mirrors76 and 78. This can significantly increase the amount of the infraredradiation 100 which is absorbed into the plate 60 and thermally coupledinto the optical resonator 12. It also significantly increases theamount of the infrared radiation 100 which is directly absorbed into theoptical resonator 12. A spacing of the mirrors 76 and 78 can be maderesonant at a particular wavelength of the infrared radiation 100 toenhance the absorption of this wavelength of the infrared radiation 100in the sensor 10. This can be done by separating the mirrors 76 and 78by a distance which is substantially equal to one-half of the wavelength(i.e. λ/2) of the infrared radiation 100. Thus, to detect infraredradiation 100 in the 8-12 μm wavelength range, the separation of themirrors 76 and 78 can be about 4-5 μm.

In the example of the thermal microphotonic sensor 10 in FIG. 8, theoptical resonator can be annular with an outer radius of 8 μm, a widthof 2.5 μm, and a thickness of 0.25 μm. The tethers can each be 0.2 μmwide and 0.25 μm thick. The silicon oxide support post 14 can be, forexample, 0.5 μm diameter and 2 μm high. The legs 68 supporting theinfrared-absorbing plate 60 and thermally coupling the plate 60 to theresonator 12 can be, for example, 0.5-1 μm in diameter and 2 μm high.The optical waveguide 20 can be about 1 μm wide and 0.2-0.25 μm thick,with the silicon oxide base 74 being 2 μm high and 0.5 μm wide.

The fourth example of the thermal microphotonic sensor 10 can befabricated on a silicon substrate 16 using conventional surfacemicromachining as will now be described with reference to FIGS. 9A-9Iwhich show a series of schematic cross-section views along the sectionline 3-3 in FIG. 8.

The silicon substrate 16 can be initially prepared by forming a thermaloxide about 0.6 μm thick over each surface of the substrate, followed bydeposition of a silicon nitride layer which can be, for example, 0.8 μmthick. The thermal oxide can be formed from the silicon substrate 16 byheating the substrate 16 to about 1050° C. in an oxygen or steamambient. The silicon nitride layer can be deposited by CVD. These layersare considered herein to be a part of the substrate 16 and are not shownin FIGS. 9A-9I.

In FIG. 9A, a layer of tungsten can be deposited on the substrate 16 toform the mirror 78. The deposition of the tungsten can be performed byCVD or alternately by evaporation or sputtering. The mirror 78 can havea thickness of 0.1-0.2 μm.

In FIG. 9B, a layer 80 of polycrystalline silicon (also termedpolysilicon) can be blanket deposited over the substrate by CVD at atemperature of about 580° C. The polysilicon layer 80, which will beused as a sacrificial material and later removed with a selectiveetchant, can be, for example, 2 μm thick. The polysilicon layer 80 canbe planarized after deposition using a CMP step. Openings 82 can then beetched through the polysilicon layer 80 at the locations where thesupport post 14 and the base 74 will be formed.

In FIG. 9C, a silicon oxide material such as TEOS or silicon dioxide canbe blanket deposited by CVD to fill in the openings 82 and to blanketthe polysilicon layer 80. A CMP step can be performed to remove any ofthe silicon oxide material which overlies the polysilicon layer 80 sothat only the silicon oxide material filling in the openings 82 is leftin place. This silicon oxide material will form the support post 14 andthe silicon oxide base 74 as shown in FIG. 9C.

In FIG. 9D, a layer of silicon nitride can be deposited over thesubstrate 16 by CVD. The silicon nitride layer, which can be 0.2-0.25 μmthick, is patterned by an RIE step to form the optical resonator 12, thetethers 18, the optical waveguide 20, the top 40 of the support post 14and the tabs 72 on the resonator 12.

Another layer 84 of polysilicon can then be blanket deposited over thesubstrate 16 by CVD and planarized by a CMP step. This polysilicon layer84 can be, for example, 2 μm thick. An RIE step can then be used to etchopenings 86 down to expose the silicon nitride tabs 72 on the resonator12 as shown in FIG. 9E.

Another layer of silicon nitride about 0.2 μm can then be blanketdeposited over the substrate 16 as shown in FIG. 9F. This siliconnitride layer can then be patterned by an RIE step to form theinfrared-absorbing plate 60. A portion of this silicon nitride layerwhich fills in the openings 86 will also form the legs 68 which supportthe plate 60 on the optical resonator 12.

In FIG. 9G, the partially-reflecting mirror 76 can be formed over theinfrared-absorbing plate 60. This can be done by depositing a0.1-μm-thick layer of aluminum using evaporation or sputtering, and thenpatterning the aluminum layer using an RIE step to form the screen.Alternately, the aluminum layer can be patterned by lift-off.

In FIG. 9H, the aluminum mirror 76 can be protected with a layer 88 ofphotoresist in preparation for removing the polysilicon sacrificiallayers 80 and 84 using a selective wet etchant comprising potassiumhydroxide (KOH). The KOH etchant will not attack the tungsten mirror 78or the various elements formed from silicon nitride, and will only veryslowly attack the silicon oxide elements 14 and 74. The thermal oxideand silicon nitride layers which blanket the silicon substrate 16protect the silicon substrate 16 from being attacked by the KOH etchant.After removal of the polysilicon sacrificial material, the photoresistlayer 88 can be removed to complete processing of the thermalmicrophotonic sensor 10 as shown in FIG. 9I. Alternate selectiveetchants which can be used to remove the polysilicon sacrificial layers80 and 84 are ethylene diamine-pyrocatechol (EDP), tetramethyl ammoniumhydroxide (TMAH), and xenon difluoride (XeF₂).

Each of the examples of the thermal microphotonic sensor 10 describedherein can be used to form individual sensors 10, or to form anone-dimensional (1-D) or two-dimensional (2-D) sensor array 90comprising a plurality of sensors 10 on a common substrate 16. Suchsensor arrays 90 have applications for infrared imaging without the needfor cryogenic cooling.

FIG. 10 shows a schematic plan view of an example of a sensor array 90formed according to the present invention. In this example of thepresent invention, a plurality of optical resonators 12 are located on acommon substrate 16, with the resonators 12 arranged in rows and columnsto form the 2-D sensor array 90. Each row of the optical resonators 12in FIG. 10 has a common optical waveguide 20. Although only a fewresonators 12 and waveguides 20 are shown in FIG. 10, one skilled in theart will understand that there can be up to one million or more opticalresonators 12 on the common substrate 16 when the sensor array 90 formsa focal plane array (FPA) for imaging infrared radiation 100. Eachoptical resonator 12 can define a pixel in the focal plane array 90,with each pixel being, for example, 5-20 μm in size. In the example ofFIG. 10, an infrared absorber (e.g. an infrared-absorbing coating 46 oran infrared-absorbing plate 60) can be provided over each opticalresonator 12 as previously described. The infrared absorber has beenomitted from FIG. 10 for clarity.

In the example of FIG. 10, light 22 from a laser 24 can be coupled intoan input optical waveguide 20′ which can be routed for coupling to a toprow of the optical resonators 12. A portion of the light 22 from theinput optical waveguide 20′ can also be split off using an evanescentwaveguide coupler 92 to feed the waveguides 20 in each additional row ofthe optical resonators 12.

To individually address each optical resonator 12 in a particular row,each resonator 12 in that row can be formed with a different resonantfrequency f₁, f₂, . . . f_(n) where n is the number of resonators 12 ineach row. This can be done by making each resonator 12 with a slightlydifferent size. The various sizes of the resonators 12 can be selectedto provide a plurality of resonant frequencies which are equally spacedapart by a frequency interval of, for example, 0.1-10 GHz. The opticalresonators 12 in each column of the sensor array 90 can have the samesize thereby providing substantially the same resonant frequency for theresonators 12 in each column. Thus, the 3×3 sensor array 90 shown inFIG. 10 can have resonators 12 of three different sizes.

To form an image of a scene of interest using the sensor array 90,infrared light 100 from the scene of interest can be imaged onto theoptical resonators 12 in the sensor array 90. This can be done using anoptical train comprising one or more lenses or mirrors which have beenomitted from FIG. 10 for clarity. A change in the resonant frequency ofeach resonator 12 due to heating of that optical resonator 12 by theinfrared radiation 100 from the scene of interest can be determined toinfer the intensity of the infrared radiation 100 to generate aninfrared image of the scene of interest.

To read out the change in the resonant frequency of each resonator 12,each column of resonators can be read out separately. This can be done,for example, by tuning or stepping the frequency f_(L) of the light 22from the laser 24 to coincide with a predetermined reference point onthe characteristic curve of each resonator 12 in a particular column ofthe array 90 and measuring the amount of the light 22 reaching thephotodetectors 30. This will allow the shift of the resonant frequencyof each resonator 12 in that column to be determined in a manner aspreviously described with reference to FIG. 2. The frequency f_(L) ofthe light 22 from the laser 24 can then be tuned or stepped to coincidewith the predetermined reference point on the characteristic curve ofeach resonator 12 in another column of the sensor array 90 and themeasurement repeated for that column of resonators 12. In this way, eachcolumn of the optical resonators 12 can be read out with the infraredradiation intensity information being stored in an information processor94 which can be a computer, a microcontroller, a digital signalprocessor, or a field programmable gate array. A referencecharacteristic curve for each resonator 12 in the sensor array 90 canalso be measured from a scan of the frequency f_(L) of the laser light22 in the absence of any incident infrared radiation 100 and stored inthe information processor 94 for reference in determining the frequencyshift of each resonator 12 due to heating from the infrared radiation100. The intensity information stored in the information processor 94can be used to form the infrared image of the scene of interest whichcan be viewed on a display 96 (e.g. a computer screen).

In the sensor array 90 of FIG. 10, the intensity information for theinfrared radiation 100 can also be determined by continuously tuning orstepping the frequency f_(L) of the light 22 from the laser 24 to scanover the characteristic curve of each resonator 12 in the array 90. Inthis way, the characteristic curve for each resonator 12, which isshifted in frequency due to heating by the infrared radiation 100, canbe measured from the light 22 exiting the optical waveguides 20 and 20′using the photodetectors 30. The characteristic curve for each resonator12 can then be compared with the reference characteristic curve for thatsame resonator 12 to determine the exact frequency shift due to heatingby the infrared radiation 100. This can be done for each resonator 12 inthe sensor array 90 using the information processor 94. From the shiftin the resonant frequency of each resonator 12 due to heating by theinfrared radiation 100, the intensity of the infrared radiation 100 canbe determined to construct the infrared image of the scene of interest.

A wavelength division multiplexing (WDM) approach can also be used toread out the intensity information for the detected infrared radiation100. In this case, which is schematically illustrated in FIG. 11, thelight 22 comprises a plurality of different wavelengths which can bespaced apart in frequency. When the number of lasers 24 is the same asthe number of resonators 12 in each row and column of the array 90 asshown in FIG. 11, then the different wavelengths of the light 22 fromthe lasers 24 can be spaced apart by about same amount (e.g. 10-50 GHz)as the different resonant frequencies f₁, f₂, . . . f_(n) of theresonators 12 in each row of the optical resonators 12.

In other embodiments of the present invention, when the number ofresonators 12 in each row and column of the array 90 is much larger thanthe number of lasers 24, then a frequency spacing of adjacent resonators12 in each row of the array 90 can be much smaller than the frequencyspacing of the different wavelengths of the light 22 from the lasers 24.As an example, when each laser 24 is scanned to interrogate 10-100resonators 12 in each row of the sensor array 90, then the resonators 12can be spaced apart in frequency by 0.1-1 GHz. In this example, eachcolumn of the resonators 12 will generally be made with substantiallythe same resonant frequency.

The different wavelengths of the light 22 can each be provided by aseparate fixed-wavelength single-frequency laser 24 (e.g. a DBR laser ora vertical-cavity surface-emitting laser operating at a wavelength near1.5 μm) which can be individually packaged, or formed on a commonsubstrate. The different wavelengths of the light 22 from the variouslasers 24 can be combined in a conventional dense wavelength divisionmultiplexer (DWDM Mux) 98, which is commonly used for fiber opticcommunications. The DWDM Mux 98 combines the light 22 from each laser 24into a single beam of light 22 containing a plurality of differentwavelengths as described above.

To make each wavelength of the light 22 from the DWDM Mux 98 tunableover at least a portion of the characteristic curve for each resonator12, an optical modulator 102 can be used. The light 22 can be fed intothe optical modulator 102 using an optical fiber (not shown), with theoptical modulator 102 being driven by a sinusoidal electrical inputsignal 104 from a voltage-controlled oscillator (VCO) 106. The VCO 106can be programmed using an input voltage from the information processor94, or from a signal generator. Alternately a digital frequencysynthesizer which can be swept in frequency over time can be used todrive the optical modulator 102. In either case, the modulator 102amplitude modulates the light 22 to generate a pair of frequency-tunablesidebands on either side of an optical carrier frequency of the light.This is schematically illustrated in FIG. 12 which shows the opticalcarrier frequencies f_(L1), f_(L2) and f_(L3) from the three lasers 24in FIG. 11. Also shown in FIG. 12 are the sidebands generated by theoptical modulator 102 at the frequencies f_(L1)±Δf, f_(L2)±Δf andf_(L3)±Δf where Δf is the frequency of the sinusoidal input signal 104.By changing the frequency Δf from the VCO 106, the sidebands can all besimultaneously tuned up or down in frequency depending upon whether thefrequency of the sinusoidal input signal 104 is added to or subtractedfrom the carrier frequencies f_(L1), f_(L2) and f_(L3). The direction oftuning of each sideband frequency is indicated by the horizontal arrowsin FIG. 12, with a range of tuning of each sideband frequency being, forexample, 5-15 GHz away from one of the carrier frequencies f_(L1),f_(L2) and f_(L3).

In the example of FIG. 11, one of the frequency-tunable sidebands fromeach laser 24 can be used to measure the frequency change (i.e.frequency shift) of each resonator 12 in a particular column of theresonators 12. The other sideband frequency and the optical carrierfrequency can be selected so that they are not located near the resonantfrequency of any of the resonators 12 and thus are not affected by anychange in the resonant frequency of the resonators 12. Alternately, anarrowband optical filter 108 can be provided in the device 10 or array90 to transmit only a single frequency-tunable sideband (see FIG. 13).In this case, the narrowband optical filter 108 can be centered about afrequency-tuning range of the sideband to be transmitted through thefilter 108 and can have a bandpass sufficiently wide so as to notappreciably attenuate the sideband over the frequency-tuning range. Atthe same time, the bandpass of the narrowband optical filter 108 can besufficiently narrow to block the transmission of the otherfrequency-tunable sideband and the optical carrier frequency. In otherembodiments of the present invention, the two frequency-tunablesidebands associated with each carrier frequency f_(L1), f_(L2) andf_(L3) can be used to measure the frequency change of two differentresonators 12 in the same row of the optical resonators 12.

Each row of the resonators 12 in FIG. 11 can be separately interrogatedwith the multi-frequency tunable light 22. This can be done by providinga splitter 110 (e.g. a conventional fiber optic splitter, or a series ofbeam-splitting mirrors) to divide the beam of light 22 from themodulator 102 into multiple beams all having about the same intensity.Each of these multiple beams of the light 22 can be directed into aseparate optical waveguide 20 as shown in FIG. 11.

The light 22, after being transmitted through the various opticalwaveguides 20, is directed into an optical switch 112 which can be usedto select the light 22 from each optical waveguide 20 in turn forfurther processing and detection. As the light 22 from a particularoptical waveguide 20 is selected by the optical switch 112, this light22 is directed into a dense wavelength division demultiplexer (DWDMDemux) 114 which spatially separates out the various sidebandfrequencies used to measure the frequency shifts of the differentresonators 12 in a particular row and directs each sideband frequency ofthe light 22 to a different photodetector 30 for detection. Anelectrical output signal 116 is generated by each photodetector 30. Thiselectrical output signal 116, which is proportional to an intensity ofthe infrared radiation 100 sensed by the optical resonators 12 viaheating thereof, can then be directed to an information processor 94(see FIG. 10), and therefrom to an optional display 96.

Electrical amplifiers, which are not shown in FIG. 11, can also beoptionally provided in the sensor array 90 of FIG. 11 to amplify theelectrical output signal 116 from each photodetector 30. Narrowbandoptical filters 108 can also be optionally provided in the sensor array90 of FIG. 11. The narrowband optical filters 108 can be used aspreviously described to transmit only one of the frequency-tunablesidebands associated with each carrier frequency, and to remove theother frequency-tunable sideband and each optical carrier frequency.

Another way of measuring the change in frequency of each opticalresonator 12 in response to heating by the infrared radiation 100 is tolinearly vary the frequency of the sinusoidal input signal 104 to theoptical modulator 102 at a known rate, and then measure the time whenthe frequency-tunable sideband of the light 22 is swept across aparticular reference point on each side of the characteristic curve ofeach optical resonator 12. This can be done initially without anyinfrared radiation 100 incident on the optical resonators 12 toestablish values of the timing for scanning across the reference pointson each side of an unshifted characteristic curve centered about theresonant frequency f₀ of each unperturbed resonator 12. These timingvalues can be stored in the information processor 94. The timingmeasurement can then be repeated with the infrared radiation 100 heatingthe optical resonators 12 to shift the resonant frequencies therein. Adifference between the timing values for each optical resonator 12 withand without the incident infrared radiation 100 can then be used tocalculate the frequency shift in each optical resonator 12 and therebydetermine the intensity of the infrared radiation 100 responsible forheating that resonator 12.

A circuit for implementing this timing measurement for a single opticalresonator 12 using a single laser 24 and a single photodetector 30 isschematically illustrated in FIG. 13. Those skilled in the art willunderstand that the circuit presented herein with reference to FIG. 13can be replicated for each photodetector 30 in the sensor array 90 ofFIG. 11.

In FIG. 13, the light 22 from the laser 24 is modulated using theoptical modulator 102 which is driven by the VCO 106. To provide alinearly-increasing input voltage 118 to the VCO 106 and thereby providea linear scan rate for tuning of the sideband frequencies of the light22 generated by the modulator 102, a signal generator 122 can be usedwith an input from a clock 124.

After the light 22 has passed through the optical waveguide 20, anarrowband optical filter 108 can be used to transmit only one sidebandfrequency of the light 22 to the photodetector 30 and to filter out theother sideband frequency and the optical carrier frequency. Theelectrical output signal 116 from the photodetector 30 can be amplifiedwith an amplifier 126 to produce an amplified signal 120 as shown inFIG. 13. The amplified signal 120 can then be input into a comparator128 where the amplified signal 120 is compared to a reference inputvoltage 130. The reference input voltage 130 has a dc voltage levelwhich defines a reference point on each side of the characteristic curvefor the resonator 12. This reference point can be, for example, 0.1-0.5of a peak voltage of the amplified signal 120. This is illustrated inFIG. 14 where the reference point is defined by the intersection of thehorizontal dashed line, which represents the reference input voltage130, with the characteristic curve of the resonator 12. Thecharacteristic curve of the resonator 12 in FIG. 14 shows the variationin the amplified signal 120 as the sideband frequency of the light 22 istuned over a range of 400 MHz.

The comparator 128 provides an output signal which will undergo a changein state whenever the difference between the amplified signal 120 andthe reference input 130 changes sign. As shown in FIG. 14, this willoccur twice—at a time designated as t₀, and again at a time designatedas t₁. A current state and a previous state of the output signal of thecomparator 128 can be sensed using a pair of sequential D-typeflip-flops 132 and 132′ which are clocked with a signal provided by theclock 124. At any point in time, a current bit representing the currentstate of the comparator 128 can be stored in the first flip-flop 132,and a previous bit representing the previous state of the comparator 128can be stored in the second flip-flop 132′. A Q output from eachflip-flop can then be combined with a Q-bar output from the otherflip-flop in an AND gate 134 as shown in FIG. 13. This provides a timingpulse from one of the AND gates 134 at the time t₀, and another timingpulse from the other AND gate 134 at the time t₁. The exact timing ofthe two timing pulses can be compared with the timing pulses obtainedfor the same resonator 12 in the absence of the infrared radiation 100and with the linear scan rate of the sideband frequencies of the light22 to determine the exact frequency shift due to heating of theresonator 12 by the infrared radiation 100. The circuit of FIG. 13 thusconverts the measurement of the frequency shift in the characteristiccurve of the resonator 12 to a timing measurement. To improve theaccuracy of this timing measurement, the timing pulses at t₀ and t₁ canbe averaged. Processing of the timing measurement information can beperformed in the information processor 94 and compared to similar timingmeasurements for the resonator 12 without any heating by the infraredradiation 100.

The matter set forth in the foregoing description and accompanyingdrawings is offered by way of illustration only and not as a limitation.The actual scope of the invention is intended to be defined in thefollowing claims when viewed in their proper perspective based on theprior art.

1. A thermal microphotonic sensor for detecting infrared radiation,comprising: an optical resonator suspended above a substrate on asupport post by at least one tether connected between the opticalresonator and the support post, with the optical resonator having aresonant frequency which changes in response to heating of the opticalresonator by the infrared radiation; an optical waveguide locatedproximate to the optical resonator to couple light from the opticalwaveguide into the optical resonator, and to transmit a remainder of thelight not coupled into the optical resonator through the opticalwaveguide; and a photodetector to receive the remainder of the lighttransmitted through the optical waveguide and to generate therefrom anelectrical output signal which is proportional to an intensity of theinfrared radiation.
 2. The apparatus of claim 1 wherein the light isprovided by a laser.
 3. The apparatus of claim 2 wherein the light ispassed through an optical modulator to amplitude modulate the light andto thereby generate a pair of frequency-tunable sidebands on either sideof an optical carrier frequency of the light.
 4. The apparatus of claim3 wherein the light is filtered with a narrowband optical filter whichsubstantially blocks transmission of the optical carrier frequency andone frequency-tunable sideband of the light, and which transmits theother frequency-tunable sideband of the light.
 5. The apparatus of claim1 wherein the optical resonator comprises a ring resonator.
 6. Theapparatus of claim 1 wherein the support post and each tether comprisesilicon oxide, and the optical resonator comprises monocrystallinesilicon.
 7. The apparatus of claim 1 wherein the optical resonator andeach tether comprise silicon nitride, and the support post comprisessilicon oxide.
 8. The apparatus of claim 1 wherein the optical resonatorincludes an infrared-absorbing coating covering at least a part of asurface of the optical resonator.
 9. The apparatus of claim 1 furthercomprising an infrared-absorbing plate supported on the opticalresonator and thermally coupled to the optical resonator.
 10. Theapparatus of claim 9 further comprising a vertical resonant cavityformed about the infrared-absorbing plate, with the vertical resonantcavity having a first mirror located on a top surface of theinfrared-absorbing plate, and a second mirror located on the substratebeneath the optical resonator.
 11. The apparatus of claim 10 wherein thefirst mirror comprises a metal screen.
 12. The apparatus of claim 1wherein the at least one tether comprises a plurality of tethers whichare interconnected by at least one concentric ring located between theoptical resonator and the support post.
 13. A thermal microphotonicsensor, comprising: at least one optical resonator supported on asubstrate; an infrared absorber supported on each optical resonator andthermally coupled to that optical resonator; an optical waveguidelocated proximate to each optical resonator to provide a coupling oflight between the optical waveguide and that optical resonator, with thecoupling of the light for each optical resonator being dependent uponheating of that optical resonator in response to infrared radiationwhich is incident onto the infrared absorber located on that opticalresonator; means for sensing the coupling of the light between theoptical waveguide and each optical resonator to provide an indication ofan intensity of the infrared radiation incident onto eachinfrared-absorbing plate.
 14. The apparatus of claim 13 wherein eachoptical resonator comprises a ring resonator.
 15. The apparatus of claim13 wherein each optical resonator which is coupled to the same opticalwaveguide has a different resonant frequency in the absence of anyheating thereof, and wherein the light in each optical waveguidecomprises a plurality of different frequencies to provide for couplingof one frequency of the light to each optical resonator.
 16. Theapparatus of claim 15 wherein each different frequency of the light istunable over a range of frequencies.
 17. The apparatus of claim 13wherein the light is provided by at least one laser, and is coupled intoan input end of the optical waveguide.
 18. The apparatus of claim 17wherein the means for sensing the coupling of the light between theoptical waveguide and each optical resonator comprises a photodetectorto sense the light emitted from an output end of each optical waveguide.19. The apparatus of claim 18 wherein the means for sensing the couplingof the light between the optical waveguide and each optical resonatorfurther comprises a laser to provide the light with a fixed frequency.20. The apparatus of claim 18 wherein the means for sensing the couplingof the light between the optical waveguide and each optical resonatorfurther comprises a laser to provide the light with a tunable frequency.21. The apparatus of claim 13 further comprising a vertical resonantcavity formed about the infrared-absorbing plate, with the verticalresonant cavity being resonant at a wavelength of the infraredradiation.
 22. A thermal microphotonic sensor, comprising: a substratehaving at least one optical waveguide formed thereon to transmit lightfrom an input end of each optical waveguide to an output end thereof; atleast one optical resonator suspended above the substrate proximate toeach optical waveguide to couple the light therebetween when a frequencyof the light is near a resonant frequency for that optical resonator; aninfrared absorber on each optical resonator to absorb incident infraredradiation and produce heat, with the heat from the infrared absorberbeing coupled into the optical resonator to change the resonantfrequency therein; and a photodetector located proximate to the outputend of the optical waveguide to detect the light and to generate anelectrical output signal containing information about an intensity ofthe infrared radiation incident onto the infrared absorber on eachoptical resonator.
 23. The apparatus of claim 22 wherein the resonantfrequency of each optical resonator coupled to the same opticalwaveguide is different.
 24. The apparatus of claim 23 wherein the lightin each optical waveguide comprises a plurality of different frequenciesto provide for coupling of one frequency of the light to each opticalresonator.
 25. The apparatus of claim 24 wherein each differentfrequency of the light is provided by a separate laser and with theplurality of different frequencies of the light being combined togetherin an optical multiplexer.
 26. The apparatus of claim 25 furthercomprising an optical modulator to amplitude modulate each differentfrequency of the light provided by each separate laser and to generate apair of frequency-tunable sidebands on either side of each differentfrequency of the light.
 27. The apparatus of claim 22 further comprisingan information processor to receive the electrical signal from eachphotodetector and to generate therefrom an image of the infraredradiation.